In recent years, demands for high performance and small size are increasing in power sources for electrical and electric equipment, electric power storage facility and electric car etc, and tendency to accomplish a high energy density is strengthened. For this reason, the lithium secondary cell using various nonaqueous electrolytes is attracting public attention.
Generally in the lithium secondary cell, a lithium metallic oxide which absorbs and desorbs lithium ion is used for its positive electrode; a carbon material which absorbs and desorbs lithium ion, a lithium metal and a lithium alloy etc. are used for its negative electrode; and a liquid electrolyte formed by dissolving a lithium salt into an organic solvent under liquid-state at ordinary temperature is used for its electrolyte. The following materials may be mentioned as the organic solvent used for the electrolyte, i.e.: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, propiolactone, valerolactone, tetrahydrofuran, dimethoxy ethane, diethoxy ethane, methoxy ethoxy ethane etc.
However, the above-mentioned organic solvents are generally apt to vaporate and have high inflammability so that they are classified into a category of flammable material. Therefore, the foregoing lithium secondary cell and particularly a comparatively large lithium secondary cell used for power sources for storing electric power and electric car etc., have had problems in securing safety in case of abuse such as over-charging, over-discharging and short-circuiting, and in securing safety under high-temperature environment.
Under such circumstances, a lithium polymer secondary cell has been developed for a lithium secondary cell excellent in its safety and partially put into practical use as the power source for electrical and electric equipment. In the lithium polymer secondary cell, the lithium metallic oxide is used for the positive electrode; the carbon material which absorbs and desorbs lithium ion, lithium metal and lithium alloy etc. are used for the negative electrode; and the polymer electrolyte which contains a lithium salt is used for the electrolyte. The polymer electrolyte mentioned in this instance is that which contains at least the lithium salt and a polymer skeleton, and generally a gel substance containing the organic solvent is further developed therefor. The following materials may be mentioned as the polymer skeleton, i.e. polyethylene oxide, polypropylene oxide, polyacrylonitrile, methyl polymethacrylate, and polyvinylidene fluoride etc.
However, problems such as worsening of cycle characteristic and charging/discharging efficiency characteristic, have occurred in the above lithium polymer secondary cell. The reason is that, because of the solid-state electrolyte, a conductivity of lithium ion in the electrolyte is smaller than that of the liquid-state electrolyte. Therefore, it is generally difficult to keep a level of the conductivity of lithium ion in the electrolyte at 1×10−3 S/cm. In addition, the gel polymer electrolyte containing the organic solvent has had problems in securing safety in case of abuse such as over-charging, over-discharging and short-circuiting etc. and in securing safety under high-temperature environment.
Under such circumstances, such nonaqueous electrolyte lithium secondary cells are proposed that do not contain the flammable substance such as the organic solvent as a main component, and is excellent in the safety. For example, in case of the nonaqueous electrolyte lithium secondary cells proposed in JP-A-4349365, JP-A-10092467, JP-A-11086905, and JP-A-11260400 etc., the lithium metallic oxide is used for the positive electrode; the carbon material which absorbs and desorbs the lithium ion, the lithium metal and the lithium alloy etc., are used for the negative electrode; and a substance containing a room temperature molten salt having quaternary ammonium organic cation and a lithium salt is used for the electrolyte. The foregoing nonaqueous electrolyte lithium secondary cells are excellent in securing the safety because the room temperature molten salt having the quaternary ammonium organic cation has almost no volatility even under its liquid-state at ordinary temperature and has inflammability or nonflammability.